A. Technical Field
This invention relates generally to optical transport networks, and more particularly to the determination of link latency within a round-trip path.
B. Background of the Invention
Optical networks are able to communicate information at high data rates. Maintaining the relative timing and sequence of this information being communicated is important for proper operation of such networks. In a multi-channel communication link, certain applications require measuring the latency difference between channels. Latency, a synonym for delay, is an expression of how much time it takes for data to get from one designated point to another. Different sources contribute to latency in different networks. As an example, in a point-to-point optical link, latency mainly includes the propagation delay, which is the time it takes for data to travel through the fiber at the group velocity of light, and the delays at intermediate nodes, which are mainly induced by the various optical components used. If the link contains optical-to-electrical-to-optical (hereinafter, “O/E/O”) sites, the latency may further include electrical processing and switching delays.
FIG. 1 illustrates a general method to measure latency difference in a multi-channel communication link. A pattern generator 110, integrated within a transmitter 100, generates a single pattern stream that is split into two pattern streams or channels at the transmitter 100 and transmits the two pattern streams down a transmission link 120 on the two channels, A1 and A2 in this example, that are being tested.
A receiving node 130, coupled to the transmission link, generates a trigger signal 140 based upon a common feature, such as an adjustment bit sequence, in each of the two pattern streams being transmitted down channels A1 and A2 respectively. An oscilloscope 150 comprising two input channels sharing the exact same time base, coupled to the receiver node 130, receives the two trigger generated signals 140 and thus measures the time delay (A1-A2) between the two pattern streams. Assuming the measurement induced additional latency difference is negligible, this measured time delay is equal to the latency difference. This measurement is meaningful when the latency difference is stable; it is feasible when the latency difference is in the delay measurement range of the oscilloscope and the fluctuation of both latency values is slow enough so that it can be followed by the oscilloscope.
Pattern streams may contain periodic features to generate periodic triggers in order to sustain the oscilloscope scans. To avoid aliasing, the period should be longer than twice the latency difference under test. However, if preexisting knowledge distinguishes which channel has a larger latency, the period only needs to be longer than the latency difference. The period can be further reduced if prior to the measurement the latency difference can be estimated to within one period.
In other fashions, the links and test setup may vary. For instance, the channels with the links can be the different WDM wavelengths on the same fiber. Also, within an optical network, the channels can be logical paths which may traverse multiple O/E/O sites. For another instance, the trigger generation may be removed if the channels are capable of transmit arbitrary patterns without any segmentation, framing, scrambling, or coding, so that pattern streams can be directly shaped into the trigger signals. In Synchronous Optical Network (hereinafter, “SONET”) however, a traffic generator must be used for pattern generation, and traffic analyzers must be used for trigger generation. The oscilloscope may be any instrument capable of measuring time delay between the two triggers or between certain features in the two pattern streams.
The aforementioned method is problematic because the test setup is divided between two nodes. Node 1 100 contains the pattern generation test setup and node 2 130 contains the trigger generation setup. Node 1 100 and node 2 130 may be stationed many miles apart, as in a submarine optical system, and if the test is not fully automated there may involve some human coordination between the node 1 100 and node 2 130.
In addition, for bidirectional measurements, both nodes must have pattern generators, trigger generators, and oscilloscope capabilities, doubling the components needed to measure latency.